After a chicken or other bird is slaughtered in a processing plant, it is carried by a leg shackle conveyor through a scalding bath, containing water at a temperature of about 128.degree. F. to 140.degree. F., to loosen the feathers prior to plucking. The immersion time is kept to the minimum necessary to loosen all the feathers, so as not to cook the outer layers of the bird, which can alter its color, among other effects. However, the feathers and anatomical irregularities both interfere with water flow around the bird and hence heat transfer, so that non-uniform scalding results. Two problem areas are around the tail, and the backs of the wings. To loosen the feathers sufficiently in the most difficult areas, it is necessary to keep the bird in the bath longer than is necessary for the better exposed areas.
In an effort to improve heat transfer, and thus reduce scalding time, it is common to agitate the water around the bird, for example by injecting hot air into the scalding tank below the conveyor line. One example of this technique is described in U.S. Pat. No. 4,566,151. When the bird is immersed in the bath, its buoyancy tends to bring it to the surface, so that part of the bird (for example, the problematic tail area) is out of contact with the hot water. Air injection has been found to aggravate the buoyancy problem.
Whenever buoyancy causes incomplete scalding, rejection of the bird, or extensive hand picking, may follow. Therefore, the buoyancy problem has been addressed previously, in a number of ways, including: (a) using a low-density scalding fluid like steam or hot air; (b) using water sprays instead of immersion--see, e.g., U.S. Pat. No. 2,412,338; (c) generating localized downward currents in a water bath sufficient to overcome buoyancy--U.S. Pat. No. 3,879,540; and (d) providing some sort of mechanical restrain above the bird. Mechanical devices previously used include rails at the surface of the bath, between which the feet of the bird pass. One such use is shown in U.S. Pat. No. 3,103,697. Techniques (c) and (d) are combined in U.S. Pat. No. 1,672,555.
A problem with physical restraints is that they generally constitute separate parts adding to the cost of the tank and requiring additional cleaning and service; they also obstruct service access to the inside of the tank. Furthermore, physical restraints above the tank provide an unnecessary opportunity for obstructing conveyors and other automated equipment. For these reasons, it would be preferable to have a scalding tank whose top was completely open, free of any obstruction.